DHCP: enhance DHCP release log

Commit 2aa23de58f added in a warning
in the event that a release failed. This would have no information
that can help anyone deal with it.

Also updated the release note to include a recommendation to use
a version of dnsmasq including dhcp_release6 on an upgrade, so
that the warning we are logging here will not happen.

Closes-bug: #1619535
Change-Id: Ia73dcf5170aaf3f874a6abe83fefb8e85b6e67e3
(cherry picked from commit d9cc6deac6)

neutron/agent/linux/dhcp.py

@@ -459,7 +459,8 @@ class Dnsmasq(DhcpLocalProcess):
         except RuntimeError as e:
             # when failed to release single lease there's
             # no need to propagate error further
-            LOG.warning(e)
+            LOG.warning(_LW('DHCP release failed for %(cmd)s. '
+                            'Reason: %(e)s'), {'cmd': cmd, 'e': e})
 
     def _output_config_files(self):
         self._output_hosts_file()

releasenotes/notes/add-dhcp_release6-ff1b8d62fd7fe76d.yaml

@@ -3,5 +3,18 @@ prelude: >
   - Call dhcp_release6 command line utility when releasing
     unused IPv6 leases for DHCPv6 stateful subnets.
     dhcp_release6 first appeared in dnsmasq 2.76
-fixes:
+upgrade:
-  - closes bug 1521666
+  - A version of dnsmasq that includes dhcp_release6 should
+    be installed on systems running the DHCP agent. Failure
+    to do this could cause DHCPv6 stateful addressing to
+    not function properly.
+  - The rootwrap filters file dhcp.filters must be updated
+    to include dhcp_release6, otherwise trying to run the
+    utility will result in a NoFilterMatched exception.
+issues:
+  - Absence of dhcp_release6 when DHCPv6 stateful addressing
+    is in use may lead to bug `1521666 <https://bugs.launchpad.net/neutron/+bug/1521666>`_.
+    Neutron supports dhcp_release6 now, but if the tool is
+    not available this leads to increased log warnings. Read
+    bug report `1622002 <https://bugs.launchpad.net/neutron/+bug/1622002>`_
+    for more details.